#include <iostream>
#include <stdio.h>
#include <cstdlib>
#include <cstring>
using namespace std;

int main(){
	int k , n , w;
	scanf("%d %d %d",&k ,&n , &w);
	int ans = (1+w)*w/2*k - n;
	if(ans < 0)
		ans = 0;
	
	printf("%d\n",ans);
	return 0;
}